The known GPS (or GNSS) localization principle uses measurements of radio signals received from a constellation of satellites.
The considerable distance of the satellites from the Earth, as well as the spread-spectrum techniques used, result in the radio signals received at the surface of the Earth having very low power (−130 dBm, or 10−16 Watts).
GPS signals are therefore very sensitive to interference (intentional or not) from other radio equipment.
The interference can for instance be the out-of-band emission of a television transmitter within the GPS frequencies, or a high-level jammer in a military operations area.
At present, known techniques exist for improving the interference tolerance of GPS receivers.
Among the known techniques, there are analog techniques or 1-bit digitizing techniques, but the most advantageous is two-bit digitizing, also called non-uniform conversion.
Two-bit digitizing makes it possible to receive GPS signals despite the presence of interference in the form of sine waves or “continuous wave” (CW) according to the terminology generally used by the person skilled in the art.
Here the implementation of two-bit digitizing is explained.
As shown in FIG. 1, a GPS localization receiver 100 is conventionally mainly constituted of three components, that is to say:                an antenna 1,        a radio frequency module 2, and        a digital module 3.        
Radio frequency module 2 is located between antenna 1 and digital module 3.
Radio frequency module 2 generally includes an interference reduction system 20.
As shown in FIG. 2, the aforementioned two-bit digitizing consists of creating, by thresholding, two bits from a scrambled GPS signal consisting of GPS signal A and interference signal B.
The first bit is called the “sign bit” and is referred to as S.
The second bit is called the “amplitude bit” and is referred to as M.
The M bit's threshold is adjustable according to the type of signal A.
As shown in FIG. 3, when signal B is of the CW type, continuous digitizing makes it possible to recover signal A from signal A+B, due in particular to the creation of bit M.
Digitizing using more than two bits is also known.
The known technique of conversion using at least two bits has advantages.
Under a strong CW type interference, and compared with an analog solution, one-bit digitizing can degrade the signal-to-noise ratio by nearly 7 dB.
On the contrary, under the same CW jammer, two-bit digitizing allows the signal-to-noise ratio to be significantly improved. The signal-to-noise ratio is then optimal if the samples for which bit M is not activated are ignored at the bit converter output, which amounts to digitizing the signal using only 1.5 bits. Thus, under a strong CW type interference, and compared with an analog solution, 1.5-bit digitizing can improve the signal-to-noise ratio by more than 10 dB, when the duty cycle of bit M is set, for example, at 20%.
Further, under gaussian interference and compared with an analog solution, two-bit digitizing allows the degradation of the signal-to-noise ratio to be limited to less than 0.6 dB, as opposed to 1.96 dB for one-bit digitizing.
Two-bit digitizing does have disadvantages, however.
There is in fact a more subtle way to jam GPS signals than to jam them with a CW type signal, whose energy forms a line in the reception spectrum of GPS signals, and whose amplitude peak is also constant. Thus one jamming solution is to adopt a gaussian jammer in a broadened hand. The energy of the jamming signal will thus spread itself through the reception spectrum of the GPS signals, with rapid fluctuations in the peak amplitude of the jamming signal, and will thus be more difficult to reject.
A technique for rejecting CW type jamming signals is known from U.S. Pat. No. 7,324,037. But this technique does not allow processing of gaussian type interference.